For Side-Imaging, there can't be anything blocking the transducer to the
left or right.
Your upper left mounting location looks like the motor mount will block
the left side.
The lower right location looks better to me.
You're smart to do a temporary transducer install and check it on
the water. You may have to move it a few times to get it the
way you want it before permanently bolting it on.
The only setting that is extremely important is the Maximum depth setting
on your Sonar menu. Too shallow and you won't see bottom.
Too deep, and your unit won't be working as fast as it could be.
You want this set slightly deeper (say 10 feet), than the water you are
currently fishing.
There are no overall best settings. Water depth, clarity, salinity, temperature,
wind/waves, boat speed, transducer location, transducer depth, transducer
mounting angle (front to back, and side to side), your boat's attitude in the water,
and your boat's hull configuration will all make a difference. You'll have to
adjust most settings while on the water. Like all of us, it will take you a little
time and tinkering to get the very best images out of your unit.
I hope you have some fun with it.